High TAN (Total Acid Number) crudes are difficult to process due to the inherent corrosion problems associated with them. Furthermore, the high naphthenic acid content in these crudes causes serious problems in desalter which leads to tight emulsion formation, more rag layer generation and incomplete separation of crude oil from wash water. This not only creates problem for processing of such crudes in downstream units but poses serious challenge for effluent treatment plant also.
Crude containing high calcium are also very difficult to process due to increase in current drawn in desalter electrostatic grid, fouling of crude preheat trains and downstream units. Calcium deposits in shell and tube of heat exchangers drastically reduce their efficiency. Calcium acts as poison to hydrotreating and FCC (fluid catalytic cracking) catalysts. Higher calcium content in vacuum residue can cause reduction of run length in delayed coker furnaces. Further, highly paraffinic crudes have problem of high pour point, higher viscosity and poor asphaltene stability. Some of the crude oils have combination of high paraffinicity, high TAN and high calcium, hence are extremely difficult to process.
High acid crude (HAC) processing has drawn considerable attention of refiners as processing of these crudes can improve the refinery profit margins if the desalter and corrosion problems associated with them are taken care of. Various methods are disclosed in the prior art to overcome the problems/difficulties associated with HAC processing. Mainly, there are two approaches to tackle the corrosion problem in high acid crude processing. First is to make the metal surface resistant to naphthenic acid corrosion either by selecting a superior metallurgy or by use of corrosion inhibitor. The second approach is to neutralize/convert the corrosive naphthenic acids in to non-corrosive products/salts. As per the first approach, several patent documents (U.S. Pat. No. 5,182,013, U.S. Pat. No. 5,252,254, U.S. Pat. No. 4,443,609, U.S. Pat. No. 4,941,994 and WO2010/119417) in the prior art explain the use of corrosion inhibitors to reduce the naphthenic acid corrosion during processing of HAC oils. However, use of corrosion inhibitor does not address other problems associated with high naphthenic acid crude processing like poor desalter performance, higher rag layer generation and the like. To tackle the corrosion issue by the second approach, prior art discloses several methods for conversion of corrosive naphthenic acids into non-corrosive products. WO/2010/121343A1 discloses the use of microorganisms (biocatalysts), or catalysts derived from these organisms (enzymes) to improve the quality of crude oil. WO/2010/118498 discloses a process for decreasing the acidity of an acidic crude oil which involves contacting an acidic crude oil with a mixture nitrogen containing compounds and in the presence of lipase enzyme.
Further, WO/2010/117403 discloses a method for thermal cracking of organic acid containing hydrocarbon feed for removal of naphthenic acids. U.S. Pat. No. 6,086,751 discloses reduction of TAN by thermal treatment. US patent application No. 20100234247 discloses a method for removal of naphthenic acid and asphaltenes using an aqueous gel. U.S. Pat. Nos. 2,795,532 and 2,770,580 discloses use of sodium hydroxide or potassium hydroxide to neutralize the naphthenic acid. U.S. Pat. No. 5,961,821 explains neutralization of high acid crudes with ethoxylated amine. U.S. Pat. No. 6,679,987 teaches use of calcium oxide or calcium hydroxide for neutralization of high acid crudes. Neutralization of naphthenic acid with base increases its hydrophilicity and thus increases the emulsion forming tendency. Consequently, demulsifier is to be used for breaking of tight emulsion if neutralization method is employed for high naphthenic acid crude processing. U.S. Pat. No. 6,599,949 teaches use of demulsifier for better separation of crude and wash water in desalter.
The prior art patent documents do not however teach a process which allows processing of high acid crude oils wherein the naphthenic acids are removed from the crude oil without any chemical treatment and/or high temperature operation. Further, processing of crudes oils having high calcium content poses serious operation problems like high current drawn in desalter grid, fouling and scaling of exchangers, coking in furnaces, poisoning of catalysts and the like. To overcome these problems, various calcium removal agents are employed to reduce the calcium content in high calcium containing crudes before desalting and distillation. Most of these calcium removing agents are organic acids. U.S. Pat. Nos. 4,778,589 and 5,078,858 teach the use of citric acid as calcium removing agent. U.S. Pat. No. 6,905,593 discloses the use of acetic acid as calcium removal agent. U.S. Pat. No. 5,660,717 discloses the uses of functionalized polyacrylic acid for removal of calcium from crude oils. U.S. Pat. No. 7,497,943 discloses the use of a-hydroxy carboxylic acid for removal of calcium from crude oils. These calcium removing agents if not removed completely from the crude oil during desalting lead to high corrosion as they are acidic compounds. Furthermore, the removal efficiency decreases with increase in calcium content of the crude oil. Twice the stoichiometric amount of organic acid is needed for each calcium cation. At higher calcium concentration, the calcium salt of removing agent become less soluble in the aqueous phase and hence their carry over with the crude oil increases.
Solvent deasphalting is a well known technique for reduction of asphaltene in atmospheric residues and vacuum residues of crude oils containing significant amount of asphaltenes. Removal of asphaltenes leads to reduction in CCR, sulfur, Nitrogen, Ni and V content as most of these impurities in crude oil are associated with asphaltenes. A wide range of deasphalting techniques are covered in the prior art documents wherein the process conditions such as extraction temperature (WO 2009/085131 A1), solvent type (U.S. Pat. No. 1,948,296), solvent to oil ratio (U.S. Pat. No. 4,290,880), use of additive with the deasphalting solvent (U.S. Pat. No. 2,587,643, U.S. Pat. No. 2,882,219, U.S. Pat. No. 3,278,415, U.S. Pat. No. 3,331,394), two step extraction (U.S. Pat. No. 2,002,004, U.S. Pat. No. 2,101,308, U.S. Pat. No. 3,074,882), type of mixing (WO 2010/044987 A2), type of contactor (US 2010/0089797 A1), contact time (US2003/0121828 A1) have been discussed in details. There are several prior art patent documents wherein the improvement in deasphalted oil (DAO) in terms of reduction in metal viz. Ni & V (U.S. Pat. No. 5,000,838, U.S. Pat. No. 4,290,880, U.S. Pat. No. 4,298,456), CCR (WO 2009/058263 A1), Sulfur (US 2009/0301931 A1), Nitrogen (US20100252483) have been disclosed. Very few patents disclose deasphalting of whole crudes. U.S. Pat. No. 4,715,946 discloses a deasphalting process of whole crude oil comprising asphaltene content of 24.1 wt % (C5 asphaltene) and 13.6 wt % (C7 asphaltene). The deasphalting process as disclosed in aforementioned US patent also include a method step of treating hydrocarbonaceous feed stream with a solvent at a temperature between 100-220° C. and at a pressure of in the range of 4-5 MPa. The process as taught in U.S. Pat. No. 4,715,946 does not disclose reduction of naphthenic acid and/or calcium from the hydrocarbon charge.
U.S. Pat. No. 5,192,421 discloses deasphalting of whole crude However, the crude oils used in the U.S. Pat. No. 5,192,421 contains very high asphaltene content (>14 wt %) and is heavy in nature having API<20. All the examples in prior art teach the solvent deasphalting of crude oils or their residues having significant amount of asphaltenes. All the prior art related to deasphalting teach that most of the impurities like metal, sulfur and nitrogen are generally associated with the asphaltene fractions. Therefore, removal of asphaltenes will remove these impurities and improve the quality of DAO. Aforesaid patent does not disclose reduction of impurities like naphthenic acids and calcium from low asphaltic crude oils (Ashpaltene content <5 wt %).
U.S. Pat. No. 5,928,501 teaches a method for simultaneous removal of naphthenic acid and sulfur from whole crude oil, wherein the asphaltene content in the whole crude oil is 2% heptane insolubles. However, U.S. Pat. No. 5,928,501 is silent on removing calcium and naphthenic acid simultaneously.
US20100163457 discloses removal of calcium from calcium naphthenate. However, US20100163457 is silent on removal of naphthenic acid and other impurities such as vanadium, nickel, sulfur, nitrogen and Conradson Carbon Residues (CCR).
Accordingly, it is desirable to provide a method to reduce the two very important impurities i.e. naphthenic acids and calcium along with other impurities (Ni, V, S, N and CCR) of whole crude oils having very low asphaltenes.